Direct measurements of the effects of inhomogeneities on the normal-state transport properties of YBa2Cu3O7-x thin films

We have used a scanning tunneling microscope to measure both the topography and potential distribution in a current-carrying polycrystalline YBa2Cu3O7-x thin film. We find steps in the potential indicative of insulating inhomogeneities in the material which occur at boundaries between clusters of grains. These results provide the first direct correlation between the microstructure and the normal-state transport properties in these materials. The implications of these results for models used to explain the nature of superconductivity in polycrystalline high-temperature superconductors are discussed.